audio experiment

main.c

#include <string.h>

#include <libopencm3/stm32/rcc.h>

#include "arm_sin_f32.h"
#include "gpio.h"
#include "i2s.h"
#include "usb.h"

static void setup_clocks(void)
{
    rcc_clock_setup_pll(&rcc_hse_12mhz_3v3[RCC_CLOCK_3V3_168MHZ]);
}

const gpio_pin led_status = {
    .gp_port  = GPIOA,
    .gp_pin   = GPIO15,
    .gp_mode  = GPIO_MODE_OUTPUT,
    .gp_level = 0
};

const i2s_config i2s3 = {
    .i2s_instance    = SPI3,
    .i2s_pll_n       = 369,
    .i2s_pll_r       = 2,
    .i2s_div         = 7,
    .i2s_mode        = I2S_MODE_MASTER_TX,
    .i2s_std         = I2S_STD_PHILIPS,
    .i2s_mclk_mode   = I2S_MCLK_ENABLED,
    .i2s_data_format = I2S_FORMAT_16,
    .i2s_pins        = {
        {
                   .gp_port = GPIOA,
                   .gp_pin  = GPIO4,
                   .gp_mode = GPIO_MODE_AF,
                   .gp_af   = GPIO_AF6,
        },
        {
                   .gp_port = GPIOB,
                   .gp_pin  = GPIO3,
                   .gp_mode = GPIO_MODE_AF,
                   .gp_af   = GPIO_AF6,
        },
        {
                   .gp_port = GPIOB,
                   .gp_pin  = GPIO5,
                   .gp_mode = GPIO_MODE_AF,
                   .gp_af   = GPIO_AF6,
        },
        {
                   .gp_port = GPIOC,
                   .gp_pin  = GPIO7,
                   .gp_mode = GPIO_MODE_AF,
                   .gp_af   = GPIO_AF6,
        },
    },
};

int main(void)
{
    setup_clocks();

    i2s_init(&i2s3);

    gpio_init_pin(&led_status);
    gpio_pin_set(&led_status);

    setup_usb();

#define SAMPLE_RATE 48000
    float f         = 1000; // sine frequency
    int   amplitude = 0x7FFF;

    int n = 0;

    while (1) {
        if (i2s_is_ready(&i2s3)) {
            int16_t sample = i2s_next_is_right(&i2s3) ? 0xCAFE : (int)(amplitude * arm_sin_f32(2.0 * PI * f * n++ / SAMPLE_RATE));
            i2s_send_sample(&i2s3, sample);
        }

        /* USB echo */
        if (usb_data_available()) {
            char buf[64] = { 0 };
            usb_read_data(buf, 64);

            switch (buf[0]) {
            case '\r':
                strncpy(buf, "\n\r", 64);
                break;

            case 0x7f:
                strncpy(buf, "\b \b", 64);
                break;

            default:
                break;
            }

            usb_send(buf, strlen(buf));
        }

        usb_poll();
    }
}